1. Field of the Invention
The invention relates to a process for producing water-redispersible polymer powder compositions having cationic functionality, to water-redispersible polymer powder compositions having cationic functionality that are obtainable by the process, and to the use thereof.
2. Description of the Preferred Embodiments
Water-redispersible polymer powders (dispersion powders) are polymer powders which are obtained in general by drying of the corresponding aqueous polymer dispersions, in the presence of a drying aid (generally a protective colloid). One of the effects of the protective colloid fraction is to prevent the polymer particles from undergoing irreversible bonding during the drying operation, because the polymer particles are enveloped by the water-soluble particles of protective colloid. A second effect of this protective colloid matrix, which dissolves again when the polymer powder is dispersed in water, is that in the aqueous redispersion, the polymer particles are present again with the particle size of the original dispersion.
Dispersion powders of this kind are used especially in chemical construction products, such as in dry mortar formulations, for example, which are made ready for use by addition of water. The addition of the dispersion powders improves the processing properties of the mortars and leads to higher mechanical strength in the set mortars.
From the prior art it is already known that protective colloids having cationic functionality are among those which can be used for producing water-redispersible polymer powders.
WO 00/05275 A1 discloses a process in which cationic monomers are polymerized in an aqueous medium and, in the presence of the polymer formed in situ in this case, further monomers are polymerized. The polymer dispersion accordingly obtained accordingly is then spray-dried in the presence of polyvinyl alcohol. A process analogous to this is described in WO 00/05283 A1, the process in this case being controlled in such a way as to form copolymer particles having heterogeneous morphology.
EP 0770640 A2 describes a process for producing dispersion powders by the drying of an aqueous polymer dispersion. A key point there is that the polymer particles in dispersion in water have a positive or negative surface charge and are dried with a drying assistant whose charge is opposite to the charge of the polymer particles in the aqueous polymer dispersion.
WO 98/13411 A1 describes the production of polymer powders by spray drying of an emulsion polymer which has anionic groups, with an amphoteric polymer of natural origin (polysaccharides, preferably casein, gelatin, and proteins) or of synthetic type (copolymers of acrylic esters and unsaturated carboxylic acids with tertiary or quaternary ammonium groups in the side chain).
WO 2007/093551 A1 describes water-redispersible polymer powders which are obtained by drying a polymer dispersion in the presence of a cationic protective colloid as drying aid. Cationic protective colloids used are polymers of monomers having quaternary ammonium groups and a halide counterion, more particularly a chloride ion. In comparison with dispersion powders dried using polyvinyl alcohol as drying aid, these powders are distinguished by better processing properties in the production of mortar compounds. The mechanical strength of the mortar compounds obtained accordingly is nevertheless deserving of improvement.